Free-space permittivity measurements of electronic materials to 1.1 THz were possible thanks to a commercial VNA and mathematics software running on a PC.
Terahertz frequencies hold great promise for short-range communications, radar, and medical applications, provided that more can be learned about the building blocks of circuits (such as substrate materials) at those high frequencies. To that end, Jonathan Hammler, Andrew J. Gallant, and Claudio Balocco of the U.K.-based School of Engineering and Computing Sciences at Durham University developed a simple system based on a vector network analyzer (VNA) for the noncontact, free-space measurement of the permittivity (dielectric constant) of materials within the 750- to 1,100-GHz frequency range.
The system can be applied to the high-throughput quality-control measurements of materials used to fabricate components—such as lenses, diffractive optics, filters, and waveguide—typically employed in terahertz applications. Main components of the measurement system include a commercial VNA and Schottky-diode-based frequency converters.
The measurement system relies on a minimum of additional components, with two parabolic mirrors and a sample holder that must be aligned with a pair of horn antennas for material measurements. System drift is minimized due to the short optical path and lack of moving components. In addition, the researchers developed a fast and easy-to-use numerical calibration method for the system.
Measurements are analyzed with the aid of the MATLAB commercial analysis software from MathWorks running on a PC to provide nonlinear optimization algorithms. Several silicon and gallium-arsenide (GaAs) wafers measurements were made, with only minimal variations around mean permittivity values for each material.
See “Free-Space Permittivity Measurement at Terahertz Frequencies With a Vector Network Analyzer,” IEEE Transactions on Terahertz Science and Technology, Vol. 6, No. 6, November 2016, p. 817.